The present invention relates generally to fluorescent lamp technology, and particularly to improved efficiency of fluorescent lamps used as backlight in, for example, AMLCD (Active Matrix Liquid Crystal Display) devices.
Light produced by a conventional fluorescent lamp is a result of excited phosphor exposed to ultra-violet (UV) light energy, e.g., generated from a mercury vapor arc stream passing through a tube having phosphor on its interior surface.
Obtaining maximum light energy output for a given power input to a fluorescent lamp used as a backlight in an AMLCD is an important operational feature. In particular, an AMLCD transmits very little of the backlight provided. For a color AMLCD, only 2.5 to 4% of the backlight passes through the AMLCD. For monochrome applications, up to 12% of the backlight passes through the AMLCD. In either case, an efficient backlight must be provided to maximize light output from the display device. The backlight produced must be as efficient as possible to maintain desired light output while minimizing power dissipation, i.e., heat generated. The lumens (light out) per watt (power in) conversion in an LCD backlight system can be taken as a measure of efficiency of a fluorescent lamp backlight system. Thus, the greater the lumens per watt conversion efficiency the more effective the fluorescent lamp device is as a backlight system in an AMLCD device.
Fluorescent lamps provide the best lumens per watt conversion efficiency relative to most practical light sources. Despite this highly efficient character of fluorescent lamps relative to other types of lighting devices, further improvement in the efficiency of conventional fluorescent backlights is desirable especially for backlighting in AMLCD applications.
According to another aspect of fluorescent backlight systems, a suitably bright and uniform light output is desired. Uniformity in light output can be obtained by significant separation between the UV light source and the phosphor coating producing visible light. For example, if the UV light source is separated by more than several feet from the phosphor, the resulting visible light issuing from the phosphor appears well distributed and uniform. Unfortunately, in many applications, including avionic display devices such as contemplated under the present invention, such separation between the UV light source and phosphor producing visible light is simply not possible. For avionic display devices, the LCD must operate in a small and highly constrained environment not well suited for producing uniform light output. As a result, many avionic display devices employing an LCD in conjunction with a backlight embodying a tubular, and possibly serpentine, fluorescent lamp suffer from a lack of uniform light output.
Fluorescent coatings, in conventional fluorescent lamp manufacturing, result from a phosphor slurry drawn into a glass tube, i.e., lamp envelope, then allowed to run out of the tube. The residual phosphor slurry material, i.e., that left on the interior walls of the glass tube, is refined through baking to remove binder material that would undesirably outgas and absorb UV light and cause a loss in light output. The result of this phosphor coating process is a moderately uniform layer of phosphor on the inside of the tube. It is known in the industry that an ideal or "optimum" phosphor coating is on the order of three to five phosphor particles thick; the average phosphor particle size being in the micro meter (10.sup.-6) range. Excitation efficiency drops for coatings thicker than the optimum thickness because some emitted light is reabsorbed within the phosphor layer, and light output efficiency falls accordingly. Likewise, phosphor coatings thinner than the optimum thickness do not capture all the potential light producing ultra-violet photons generated by the mercury arc stream. Light output is then less than that possible for the amount of power provided to the lamp in producing the arc. As used herein, the terms "relatively thin" and "relatively thick" presented in reference to a phosphor coating shall refer to the thickness of the phosphor coating as being either thinner or thicker, respectively, than the above-noted "optimum" phosphor coating thickness.
The prevailing rule for manufacturing fluorescent lamps is that relatively thin phosphor coatings are better and more economical than relatively thick phosphor coatings. High volume manufacturing processes will not support an optimum phosphor coating thickness. Because phosphor coatings tend to be slightly less than optimum, i.e., relatively thin, there is a portion of UV light energy not absorbed by the phosphor coating. The energy contained in the unabsorbed UV light represents a loss or inefficiency of the system because the unabsorbed UV light is not used by the phosphor to produce fluorescence.
The process for creating a compact fluorescent lamp light source for a backlight in LCD devices further compounds the problems of non-uniformity and inefficiency, i.e., loss of UV photons, for fluorescent lamps. In conventional LCD backlighting systems, a serpentine configuration is provided by bending a straight fluorescent lamp, i.e., usually bending a fluorescent lamp tube having an interior phosphor coating in place. Under such method of manufacture, it is difficult or impossible to provide a uniform phosphor coating on the inside of the bent tube. First, to bend the lamp it is necessary to heat the lamp to very near the melting point of the glass tube. Exposure of the phosphor coating to this high heat degrades the phosphor coating, and thereby causes inefficiency with respect to energy applied to the lamp. Second, bending the lamp increases the length of the tube on the outside of the bend and decreases the length on the inside of the bend. This stretching and compressing of the glass tube causes thinning and thickening, respectively, of the phosphor coating relative to the phosphor coating in the straight portions of the tube. Consequently, when the lamp is illuminates the bent regions are darker than the straight portions of the lamp causing additional non-uniformity in light output.
It is desirable, therefore, that a fluorescent lamp as a backlight for an LCD be more efficient with respect to the utilization of the available ultraviolet light by the phosphor coating to produce visible light. Furthermore, it is desirable that a fluorescent lamp used as a backlight in an LCD produce a uniform output in a size-constrained device such as an avionic flight display device.
The subject matter of the present invention addresses these concerns of the prior fluorescent lamp arrangements and provides a more efficient and more uniform light output for a fluorescent backlight in an LCD device especially as applied to an avionic display instrument.